Answer:
Mitochondria
Step-by-step explanation:
According to this question, treadmill training has been researchfully shown to increase the number of certain energy releasing structures in the brain cells of rats. The cell consists of certain structures that play specific roles called ORGANELLES.
One of these organelles called MITOCHONDRION is the one responsible for synthesizing energy in the cell. Hence, the mitochondrion is the structure being shown/depicted as the energy releasing structures in the rat's brain cells.